Generally, fuel cells are under development to replace existing fossil fuels with eco-friendly energy. Fuel cells, such as home fuel cells for supplying electric power to home, fuel cells for electric vehicles, fuel cells for portable terminals and notebook computers, and fuel cells for supplying electric power in a portable manner at home, are being studied and developed in various application fields.
Fuel cells are typically classified, according to a type of electrolyte, into phosphoric acid fuel cell (PAFC), alkaline fuel cell (AFC), proton exchange membrane fuel cell (PEMFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC), direct methanol fuel cell (DMFC), direct borohydride fuel cell (DMFC) and the like.
Operating temperatures of these fuel cells are usually about 200° C. for the phosphoric acid type, 60-110° C. for the alkali type, room temperature to 80° C. for the polymer electrolyte type, 500 to 700° C. for the molten carbonate electrolyte type, high temperature over 1000° C. for the solid oxide type and 25 to 130° C. for the direct methanol type.
In most cases, unlike typical batteries (secondary batteries), a fuel cell supplies fuel (hydrogen gas or hydrocarbon) and oxygen to a fuel electrode (anode) and an air electrode (cathode), respectively, which form a stack, thereby inducing an electrochemical reaction between the hydrogen and the oxygen without a burning (oxidation) reaction. An energy difference between states before and after the electrochemical reaction is then directly converted into electric energy.